1. Field of the Invention
This invention relates mass spectrometers and more specifically to mass spectrometers of the non-scanning type.
2. Discussion of the Background
Mass spectrometers measure the atomic mass of ions by separating ions of different charge-to-mass ratios using a combination of an electric field and a magnetic field to bend the ions proportional to their charge-to-mass ratio. Previous mass spectrometers have been large and heavy, typically over 1,000 lbs., because of their requirements for a very high pumping speed in order to differentially pump an ionizing region. The ionizing region generates ions of the molecules to be measured in a mass spectrometer detection region and generates an ion beam comprised of those molecules for transmission of the ions to the detection region. The ion beam must have a very low density so that ions in the ion beam do not collide with one another thereby perturbing their trajectories. Furthermore, the initial ion beam trajectories must be highly collimated so that they do not collide with walls along the ion beam path and so that the ions can be mass separated.
Prior mass spectrographs were also large because their magnetic sectors were designed to widely spatially separate ions of different masses in order to analyze those masses. In particular, the magnetic sectors used large magnets or electromagnets.
In other prior devices the ion trajectories were changed such that ions of only one mass to charge ratio passed through a fixed slit and their flux was measured by a detector located downstream from the slit. The fluxes of different mass ions were thus measured in succession. A mass spectrometer using a single slit is called a scanning mass spectrometer.
In a non-scanning mass spectrometer, ions of different masses are spatially separated and can be measured simultaneously. Consequently, a non-scanning mass spectrometer possesses a higher sensitivity and is capable of a higher data acquisition rate than a scanning mass spectrometer.
It has been found very useful to combine gas chromatography as a means for separating components of a mixture of gases prior to their introduction into an ion source of a mass spectrometer. This has been accomplished by providing a relatively long gas chromatographic column on the order of 30 meters whose input end receives gases whose mass spectra are to be determined and whose output end expels gases separated in time into an ionization chamber. Such an instrument is called a GC-MS. A 30 meter long gas chromatography tube has been required because standard tubes which have inner diameters of greater than 200 microns and therefore require a relatively long time to perform their component separation function. A gas chromatographic tube separates a mixture of multiple gases flowing down the tube into single components which exit from the tube at different times. Separation of components by chromatography makes the measurement of their individual mass spectra simpler. Because of the length and rather large inner diameter of gas chromatography tubes, the duration of exit of any given bunch of similar molecules from these tubes has been rather long, typically on the order of a few seconds. Because of the large tube diameter, a relatively large volume of gas flows through these tubes, typically 2 to 5 atm cm.sup.3 per minute. The large volume of gas flowing through these tubes requires a very large pump at or past the ionization chamber in order to provide the very low pressure and density necessary for an ion beam for a mass spectrometer.
It has been demonstrated that microbore capillary tubes, i.e., capillary tubes whose inner diameter is less than 100 microns, may be advantageously used in gas chromatography. These tubes greatly reduce the volume of gas flowing through them while providing the necessary separation of different types of molecules flowing through them. For example, a 50 micron inner diameter, 3 meter long microbore tube has a gas flow rate of 0.02 atm cm.sup.3 per minute. Because of the reduced gas load the size and weight of vacuum pumps necessary to reduce the pressure to that suitable for mass spectrometry is greatly reduced. Pumps weighing less than 20 pounds are adequate. However, molecules of a similar type exit a microbore capillary gas chromatography tube in a much reduced time period compared to normal GC tubes. Molecules of a similar type typically exit a microbore gas chromatography tube in a time period of a fraction of a second. When a microbore capillary gas chromatography tube is hooked up to a scanning mass spectrometer to form a GC-MS, ions generated from one type of molecule impinge upon the image plane of a mass spectrometer for a period of time approximately equal to the time period during which those ions exit the microbore capillary tube, i.e., a fraction of a second. Unless such a scanning mass spectrometer scans its entire mass detection range in a time faster than the duration of a pulse of ions exiting a microbore tube, those ions may remain undetected by the mass spectrometer. Furthermore, multiple mass spectral scans must be made in this brief period for the components to be quantified.
To solve this problem, it has been shown that a microbore capillary tube may be used to provide source gas to a non-scanning mass spectrometer. A non-scanning mass spectrometer functions by simultaneously detecting ions along the length of an image plane in order to simultaneously determine the content of the ion beam for ions of different masses.
Existing non-scanning mass spectrometers typically weigh at least several hundred pounds because of the high pumping requirements requiring large pumps to provide high pumping capacity and because of the large magnets used in the magnetic sectors of these devices. Therefore, these devices are unsuitable for routine transportation or for routine field testing. Existing non-scanning mass spectrometers have had magnetic flux densities in the gap between their pole pieces of 10 kilogauss or less. However, the 10 kilogauss values achieved by prior non-scanning devices required very thick magnets and very thick yokes to conduct sufficient magnetic flux. Prior non-scanning devices used magnetic materials having energy products of below 7.times.10.sup.6 GOe.